<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8" />
    <title>无标题文档</title>
    <style>
        body{margin:0; padding:0}
        #to_top{width:30px; height:40px; padding:20px; font:14px/20px arial; text-align:center;  background:#06c; position:absolute; cursor:pointer; color:#fff}
    </style>

    <script>
        window.onload = function(){
            var oTop = document.getElementById("to_top");
            var screenw = document.documentElement.clientWidth || document.body.clientWidth;
            var screenh = document.documentElement.clientHeight || document.body.clientHeight;
            oTop.style.left = screenw - oTop.offsetWidth +"px";
            oTop.style.top = screenh - oTop.offsetHeight + "px";
            window.onscroll = function(){
                var scrolltop = document.documentElement.scrollTop || document.body.scrollTop;
                oTop.style.top = screenh - oTop.offsetHeight + scrolltop +"px";
            }
            oTop.onclick = function(){
                document.documentElement.scrollTop = document.body.scrollTop =0;
            }
        }

    </script>
</head>

<body style="height:1900px;">

<h1>返回顶部</h1>


<div id="to_top">返回顶部</div>
</body>
</html>